通过以前的文章，我们了解到@Table、@Column、@Id实现了单表的映射，并且书剑有一个@Id指定的唯一字段。有时我们的数据表也许是有多个主键联合组成的，因此对于单表映射的主键，还可以进行如下两种联合主键映射。 联合主键：使用@IdClass指定外部主键 联合主键：使用@EmbeddedId嵌入外部主键下面通过实例来看这两种主键的开发方法。Demo：设计一个家庭表Family的数据结构
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（一）联合主键：使用@IdClass指定外部主键
步骤： 1、建立一个主键类：类中对应了主键字段 2、在实体Bean中通过@IdClass注释符引用该类
以实现外部主键的引用。
1）新建外部主键类FamilyPK.java由于Family中设置联合主键man和woman，因此外部主键类FamilyPK也需要定义两个同样的变量，并添加一个以这两个变量为输入的构造函数，同时添加getter/setter函数。作为外部主键需满足：1.必须实现Serializable接口2.必须有默认的public无参数的构造方法3.必须覆盖equals()和hashCode()方法。equals()方法用于判断两个对象是否相同，EntityManager通过find()方法来查找实体，是根据equals()的返回值来判断的。本例中，只有对象的man和woman值完全相同或属于同一个对象时才返回true，否则返回false。hashCode()方法返回当前对象的哈希码。生成的hashCode()相同的概率越小越好，，算法可以进行优化。package com.tgb.itoo.exam.entity;import java.io.Serializable;@SuppressWarnings("serial")public class FamilyPK implements Serializable {private String man;//丈夫private String woman;//妻子public String getMan() {return man;}public void setMan(String man) {this.man = man;}public String getWoman() {return woman;}public void setWoman(String woman) {this.woman = woman;}public FamilyPK() {}public FamilyPK(String man, String woman) {this.man = man;this.woman = woman;}@Overridepublic int hashCode() {final int prime = 31;int result = 1;result = prime * result + ((man == null) ? 0 : man.hashCode());result = prime * result + ((woman == null) ? 0 : woman.hashCode());return result;}@Overridepublic boolean equals(Object obj) {if (this == obj)return true;if (obj == null)return false;if (getClass() != obj.getClass())return false;FamilyPK other = (FamilyPK) obj;if (man == null) {if (other.man != null)return false;} else if (!man.equals(other.man))return false;if (woman == null) {if (other.woman != null)return false;} else if (!woman.equals(other.woman))return false;return true;}}2）使用@IdClass在实体Bean类Family.java中指定外部主键。通过注释符来设置与表、字段的映射关系。注意，该实体中需要标注联合主键：1、在man和woman的getter函数前都添加@Id注释符，表示都是主键2、在类名钱使用@IdClass引用外部主键类package com.tgb.itoo.exam.entity;import java.io.Serializable;import javax.persistence.Column;import javax.persistence.Entity;import javax.persistence.Id;import javax.persistence.IdClass;import javax.persistence.Table;@SuppressWarnings("serial")@Entity@Table(name="family")@IdClass(FamilyPK.class)public class Family implements Serializable {private String man;// 丈夫private String woman;// 棋子private String address;// 地址@Idpublic String getMan() {return man;}public void setMan(String man) {this.man = man;}@Idpublic String getWoman() {return woman;}public void setWoman(String woman) {this.woman = woman;}@Column(name="address" ,length=100)public String getAddress() {return address;}public void setAddress(String address) {this.address = address;}}3）新建远程接口类FamilyDAORemote.java定义两个接口：新增、根据主键查询package com.tgb.itoo.exam.service;import javax.ejb.Remote;import com.tgb.itoo.exam.entity.Family;@Remotepublic interface FamilyDAORemote {// 新增public boolean insert(Family family);// 插入public Family selectByPK(String man, String woman);}4）开发实现类FamilyDAO.java1.首先构造一个主键对象FamilyPK2.后调用find()方法根据该主键对象进行查询package com.tgb.itoo.exam.papermanage.serviceimpl;import javax.ejb.Stateless;import javax.persistence.EntityManager;import com.tgb.itoo.exam.entity.Family;import com.tgb.itoo.exam.entity.FamilyPK;import com.tgb.itoo.exam.service.FamilyDAORemote;@Statelesspublic class FamilyDAO implements FamilyDAORemote {protected EntityManager em;@Overridepublic boolean insert(Family family) {try {em.persist(family);} catch (Exception e) {e.printStackTrace();return false;}return true;}@Overridepublic Family selectByPK(String man, String woman) {FamilyPK epk = new FamilyPK(man, woman);return em.find(Family.class, epk);}}
5）测试：客户端调用
不要因为世态变迁而埋怨,不要因为命运多舛而怨恨.
